Method of producing a protective gas and a gas mixture therefor

ABSTRACT

The present invention relates to a method of producing a protective gas consisting of 0.0010-0.100 percent by volume nitric-oxide, 0-90 percent by volume helium, 0-5 percent by volume hydrogen, 0-50 percent by volume carbon dioxide, and the remainder argon. The invention also relates to a gas mixture for producing the productive gas. The method of producing the protective gas is characterized by dissolving nitric oxide in condensed argon to a concentration of 0.0010-0.100 percent by volume, calculated on gas phase, vaporizing the resultant solution and mixing remaining constituents in the vaporized solution in such proportions as to obtain a protective gas of the predetermined composition. The gas mixture used for producing the protective gas is characterized in that it consists of 0.0010-0.100 percent by volume nitric oxide in condensed argon, calculated on gas phase, and is preferably held under an overpressure.

BACKGROUND OF THE INVENTION

The present invention relates to a method of producing a protective gasconsisting of 0.0010-0.100 percent by volume of nitric oxide, 0-90percent by volume helium, 0-5 percent by volume hydrogen, 0-50 percentby volume carbon dioxide and the remainder argon, and also to acondensed gas mixture for the production of said protective gas.

In electric gas-arc welding processes there is often used a protectivegas in the form of a gas mixture consisting of or comprising argon andnitric oxide. The protective gas mixture or protective gas may alsoinclude carbon dioxide, helium and/or hydrogen. To large consumers ofprotective gases, these gases containing nitric oxide, there is atpresent delivered a gas pre-mixture, a master gas, consisting of anitric oxide in argon under high pressure, this gas being delivered inconventional steel bottles. The proportion of nitric oxide present isnormally from 1-5 percent by volume, for instance 1.8 percent by volume.The desired protective gas is then produced by mixing the various gaseswith the gas pre-mixture to the composition desired. The amount ofnitric oxide present in the final protective gas is normally at most0.030 percent by volume.

Since nitric oxide is a poisonous gas, it is desirable to use lowerproportions of nitric oxide in the master gas than those used hitherto.This results in higher transportation costs, since it is necessary totransport to the consumer more gas bottles containing master gas atunchanged protective gas consumption.

Another drawback with the master gas is that when preparing a protectivegas it is necessary to supply additional argon in order to reach thedesired final concentration of argon in the protective gas produced.

OBJECTS OF THE INVENTION

Accordingly, one object of the present invention is to provide acondensed gas mixture of nitric oxide in argon with a lower proportionof nitric oxide than has hitherto been used in the master gas withoutincreasing the transportation costs as a result hereof and withoutoccasioning excessive switching between gas sources (nitric oxidecontaining gas).

Another object of the present invention is to provide a condensed gasmixture of nitric oxide and argon for the production of a protective gaswhich does not require the additional admixture of pure argon.

Still another object of the invention is to provide a gas mixture ofnitric oxide and argon in liquid form in order to considerably reducethe number of gas-mixture deliveries to the consumer.

A further object of the invention is to provide a method for producing aprotective gas which in addition to containing nitric oxide and argonalso contains helium, carbon dioxide and/or hydrogen, by using simplermixing apparatus.

SUMMARY OF THE INVENTION

These objects are achieved with a condensed gas mixture which comprisescondensed argon in which 0.0010-0.100 percent by volume, calculated ongas phase, of nitric oxide is dissolved. Argon is a liquid between about-185.9 and -189.2 degrees Celsius at atmospheric pressure, whereasnitric oxide, NO, has a freezing point at about. -164 degrees Celsius.

It has now surprisingly been found that nitric oxide can be dissolved inliquid argon at atmospheric pressure in an amount of at least up to 0.1percent by volume calculated on gas phase, and that the vaporized gasmixture has essentially the same composition as the liquid phase.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described in more detail with reference to anexemplifying embodiment thereof illustrated in the accompanying drawing,the single FIGURE of which is a graph in which the nitric oxideconcentration, in ppm, is plotted as a function of time during whichremovals of gas mixtures takes place.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A pressure container which accommodates about 800 kg liquid argon wasfilled with liquid argon, whereafter nitric oxide was dissolved in theargon until the nitrogen monoxide concentration had risen to about 300ppm. This dissolution can be achieved by dissolving in condensed argon amaster gas consisting of nitric oxide in argon, e.g. 1.8 percent byvolume NO in argon. 60 liters of the gas mixture were taken from thecontainer each minute over a period of 8 hours in each working day. Thisgas outtake was intended to simulate the consumption of protective gasin a arc welding process. The weight of the gas mixture remaining in thecontainer, both in liquid phase and gas phase, is given in the FIGURE.The experiment was continued for 30 calendar days. Samples of thecomposition of both the and liquid phases were continuously taken. Theweight of the remaining gas mixture is plotted on the right-handabscissa axis. The FIGURE also shows the concentrations of nitric oxidein argon as a function of time. The concentration of nitric oxide isplotted in ppm on the left-hand abscissa axis of the graph. It will beseen from the graph that the nitric oxide concentration in the gas phaseis almost equal to the nitric oxide concentration in the liquid phase.Furthermore, it is evident that the concentrations in the two phases aresubstantially independent of time and of the amount of condensed gasmixture in the container. The container was fitted with a pressuresafety valve which opened automatically when the pressure in thecontainer had risen to 17.5 bars overpressure. The pressure variedbetween about 3 bars and 17.5 bars during the experimental period.

Thus, when producing a protective gas consisting solely of argon andnitric oxide, the predetermined quantity of nitric oxide can bedissolved in the liquid argon and then taken out either in vapor phaseor liquid phase, it being necessary in this latter case to vaporize themixture in liquid phase.

When producing a protective gas consisting of 300 ppm nitric oxide, 20percent by volume carbon dioxide, 30 percent by volume helium and theremainder argon for instance, there is prepared a master gas whichconsists of 600 ppm nitric oxide in liquid argon. This mixture, whichconstitutes 50 percent by volume of the protective gas, is then mixedwith 20 percent by volume carbon dioxide and 30 percent by volumehelium. This requires the use of a gas mixer which has three inputs. Onthe other hand, when working on the basis of a known gas mixture ofnitric oxide in argon in gas form with a nitric oxide concentration of1.8 percent, it will be necessary to admix pure argon in order to obtainthe desired protective gas. This would require the use of a gas mixerequipped with four inputs.

The present invention enables the use of a cryogenic liquid containerwhich is filled with condensed argon in which at most 0.1 percent byvolume nitric oxide is dissolved. Available commercially are transportcontainers for cryogenic liquids which accommodate a gas quantitycorresponding to more than 40 large steel bottles (40 liters) at apressure of 200 atm. In this case, the large consumer does not require atank for pure argon, but need only have access to those gases whichshall be included in the protective gas. One advantage with theinventive master gas in condensed form is that no gas mixer is requiredwhen the protective gas shall only contain argon ill addition to nitricoxide. In other cases a simple, and therewith less expensive gas mixercan be used, since it is no longer necessary to add argon in order toobtain a protective gas of the predetermined composition.

According to the present invention an advantage is afforded when theliquid phase is taken from the argon-nitric oxide mixture in thecontainer and vaporized before use as a protective gas, or admixed withother protective gas constituents. According to another preferredembodiment, both gas and liquid phases are taken from the container, andthe latter phase vaporized prior to combining it with the gas phase.

An advantage is also afforded when the cryogenic gas container is apressure vessel fitted with a pressure valve, such as to prevent thebuild-up of non-permitted high pressures in the container. The valveshould be set to a relatively high pressure, e.g. a pressure of 20 bars.It has also been found that the gas pressure above the liquid in thecontainer should be 3 bars or more, in order to obtain a gas phase ofapproximately the same composition as the liquid phase. Since thecryogenic gas container is not normally constructed for gas pressureshigher than 25 bars, the preferred gas pressure in the container is from3-25 bars, particularly 5-18 bars. An overpressure valve which opens ata lower pressure than 3 bars is not recommended, since when the mastergas is not used, for instance over the weekend, the pressure willquickly rise to 3 bars and all gas formed thereafter and resulting in ahigher pressure than 3 bars will be lost to the surroundings.

We claim:
 1. A method of producing a protective gas comprising0.0020-0.100 percent by volume nitric oxide, 0-90 percent by volumehelium, 0-5 percent by volume hydrogen, 0-50 percent by volume carbondioxide, and the remainder argon, characterized by dissolving nitricoxide in condensed argon to a concentration of 0.0010-0.100 percent byvolume, calculated on gas phase, vaporizing the resultant solution andadmixing remaining constituents in the vaporized solution in suchproportions as to obtain a protective gas of predetermined composition.2. A product for producing a protective gas consisting of 0.0010-0.100percent by volume nitric oxide, 0-90 percent by volume helium, 0-5percent by volume hydrogen, 0-50 percent by volume carbon dioxide, andthe remainder argon, characterized in that said product consists ofliquid argon in which 0.0010-0.100 percent by volume nitric oxide,calculated on gas phase, is dissolved.
 3. A product according to claim2, characterized in that the condensed gas mixture is held under anoverpressure.
 4. A product according to claim 3, characterized in thatthe overpressure is 3-25 bars, particularly 7-18 bars.
 5. A method forproducing a protective gas comprising argon and a predetermined amountof nitric oxide, said predetermined about being between 0.001 and 0.100volume percent based on the volume of said protective gas, said methodcomprising dissolving said predetermined amount of nitric oxide inliquid argon and thereafter vaporizing the solution so obtained to formsaid protective gas.
 6. The method of claim 5 wherein said solution isvaporized at a pressure of about 3 to 25 bar.
 7. The method of claim 6wherein said solution is vaporized at a pressure of about 5 to 18 bars.8. The method of claim 5 wherein said protective gas further comprises apredetermined amount of helium between 0 and 90 percent, a predeterminedamount of hydrogen between 0 and 5 percent and a predetermined amount ofcarbon dioxide between 0 and 50 percent, the percents being based on thevolume of said protective gas, said method further comprising admixingsaid predetermined amounts of helium, hydrogen and carbon monoxide withthe vaporized solution of argon and nitrogen oxide to form saidprotective gas.
 9. The method of claim 8 wherein said protective gascontains at least one of helium, hydrogen and carbon monoxide. 10.Liquid argon containing 0.001 to 0.100 mole percent nitric oxidedissolved therein.
 11. The liquid argon of claim 10 consisting of argonand nitric oxide.